Add LLMCheckerChain (#281)

Implementation of https://github.com/jagilley/fact-checker. Works pretty
well.

<img width="993" alt="Screenshot 2022-12-07 at 4 41 47 PM"
src="https://user-images.githubusercontent.com/101075607/206302751-356a19ff-d000-4798-9aee-9c38b7f532b9.png">

Verifying this manually:
1. "Only two kinds of egg-laying mammals are left on the planet
today—the duck-billed platypus and the echidna, or spiny anteater."
https://www.scientificamerican.com/article/extreme-monotremes/
2. "An [Echidna] egg weighs 1.5 to 2 grams (0.05 to 0.07
oz)[[19]](https://en.wikipedia.org/wiki/Echidna#cite_note-19) and is
about 1.4 centimetres (0.55 in) long."
https://en.wikipedia.org/wiki/Echidna#:~:text=sleep%20is%20suppressed.-,Reproduction,a%20reptile%2Dlike%20egg%20tooth.
3. "A [platypus] lays one to three (usually two) small, leathery eggs
(similar to those of reptiles), about 11 mm (7⁄16 in) in diameter and
slightly rounder than bird eggs."
https://en.wikipedia.org/wiki/Platypus#:~:text=It%20lays%20one%20to%20three,slightly%20rounder%20than%20bird%20eggs.
4. Therefore, an Echidna is the mammal that lays the biggest eggs.


cc @hwchase17

docs/examples/chains/llm_checker.ipynb

@@ -0,0 +1,58 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# LLMCheckerChain\n",
+    "This notebook showcases how to use LLMCheckerChain."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.chains import LLMCheckerChain\n",
+    "from langchain.llms import OpenAI\n",
+    "\n",
+    "llm = OpenAI(temperature=0.7)\n",
+    "\n",
+    "text = \"What type of mammal lays the biggest eggs?\"\n",
+    "\n",
+    "checker_chain = LLMCheckerChain(llm=llm, verbose=True)\n",
+    "\n",
+    "checker_chain.run(text)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.12"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}